Fate, Transport, and Retention of Applied 15N Labelled Nitrogen in Forest Soils

Applied 15N labelled ammonium was retained longer in an acid soil with moder humus but was quickly nitrified and leached as nitrate in base rich soils with mull humus type of surface organic layer. Longer retention of applied ammonium in acid soils was associated with low nitrification (heterotrophic type) rate. However,the surface organic layer in moder humus adsorbed only a small amount of the applied ammonium whereas most was adsorbed in the mineral soil layers.Retention of ammonium by adsorption depended on soil temperature which was reduced at higher temperature by increasing nitrification, and was increased with
decreasing temperature by lowering nitrification. This mechanism potentially delayed leaching losses of applied ammonium as nitrates in acid soils especially at low temperatures in the nongrowing season and probably increased the uptake of N by microbes and plants.
Retention of 15N labelled ammonium in organic compounds (microbial biomass,soil organic matter) was high when microbial biomass was high. The dominant effect of microbial immobilisation, which was independent of soil layers and humus forms, suggested the importance of the fate of microbial detritus for the long-term retention of deposited ammonium in forest soils. Retention of 15N labelled nitrate in organic compounds was very low in the Solling soil in contrast to other studies with different application procedures of labelled nitrate.